Can Twitter help predict epidemics?

Twitter users send around 500 million tweets a day, an endless fire hose of information about how people feel, what they're doing, what they know and where they are. For epidemiologists and public health officials, it's a potential gold mine of data, a possible way to track where disease is breaking out and how it spreads, as well as how best to help—but only if they can figure out how to find the useful signal amid all that noise.